man Proc::PID::File () - a module to manage process id files


Proc::PID::File - a module to manage process id files


  use Proc::PID::File;
  die "Already running!" if Proc::PID::File->running();


This Perl module is useful for writers of daemons and other processes that need to tell whether they are already running, in order to prevent multiple process instances. The module accomplishes this via *nix-style pidfiles, which are files that store a process identifier.

Module Interface

The interface consists of a single call as indicated in the Synopsis section above. This approach avoids causing race conditions whereby one instance of a daemon could read the pidfile after a previous instance has read it but before it has had a chance to write to it.

running [hash[-ref]]

This method receives an optional hash (or, alternatively, a hash reference) of options, which determines function behaviour.

The returns value is true when the calling process is already running. Please note that this call must be made *after* daemonisation i.e. subsequent to the call to fork().

The options available include the following:

Specifies the directory to place the pid file. If left unspecified, defaults to /var/run.
Indicates the name of the current process. When not specified, defaults to basename($0).
verify = 1 | string
This parameter helps prevent the problem described in the WARNING section below. If set to a string, it will be interpreted as a regular expression and used to search within the name of the running process. A 1 may also be passed, indicating that the value of $0 should be used (stripped of its full path). If the parameter is not passed, no verification will take place. Please note that verification will only work for the operating systems listed below and that the os will be auto-sensed. See also DEPENDENCIES section below. Supported platforms: Linux, FreeBSD
Turns debugging output on.


Erick Calder <>


1k thx to Steven Haryanto <> whose package (Proc::RID_File) inspired this implementation.

Our gratitude also to Alan Ferrency <> for fingering the boot-up problem and suggesting possible solutions.


For Linux and FreeBSD, support of the verify option (simple interface) requires the ps utility to be available. This is typically found in the procps RPM.


This module may prevent daemons from starting at system boot time. The problem occurs because the process id written to the pidfile by an instance of the daemon may coincidentally be reused by another process after a system restart, thus making the daemon think it's already running.

Some ideas on how to fix this problem are catalogued below, but unfortunately, no platform-independent solutions have yet been gleaned.

- leaving the pidfile open for the duration of the daemon's life
- looking at /proc/$PID/stat for a process name
- check mtime of the pidfile versus uptime; don't trust old pidfiles
- try to get the script to nuke its pidfile when it exits (this is vulnerable to hardware resets and hard reboots)
- try to nuke the pidfile at boot time before the script runs; this solution suffers from a race condition wherein two instances read the pidfile before one manages to lock it, thus allowing two instances to run simultaneously.


For help and thank you notes, e-mail the author directly. To report a bug, submit a patch or add to our wishlist please visit the CPAN bug manager at:


The latest version of the tarball, RPM and SRPM may always be found at: Additionally the module is available from CPAN.


This utility is free and distributed under GPL, the Gnu Public License. A copy of this license was included in a file called LICENSE. If for some reason, this file was not included, please see to obtain a copy of this license.

$Id:,v 1.16 2004/04/08 02:27:25 ekkis Exp $